Receivers in bipolar pulse code modulated (PCM) telephone systems frequently use the received data pulses to generate an internal clock in synchronism with the received pulse train. In modern high-speed equipment, the required close synchronization tolerances make it necessary for the pulse train to contain a "1" in at least every eighth bit interval. To achieve this result, modern PCM encoders use bipolar eight-zero suppression (B8ZS).
B8ZS works by detecting any sequence of eight consecutive zeros in the pulse train and replacing it with a standardized bit pattern of ones and zeros which contains specific bipolar violations and is therefore recognizable as an artifact indicating eight consecutive zeros when the pulse train is decoded.
Unfortunately, the integrity of a telephone PCM span line is normally tested, at each successive span terminal or span line switch along the line, by counting, i.a., bipolar violation errors. If the error rate exceeds a predetermined threshold, the affected span is automatically taken out of service and replaced with a spare span, as described in U.S. Pat. Nos. 3,983,340 and 4,042,794.
In order to avoid the unnecessary switching of a functioning span by pulse trains containing a large number of B8ZS patterns, it is necessary for span line switches to ignore bipolar violations in B8ZS patterns, but to count all other bipolar violations in the received pulse train as errors.